In 2002, Lainey causes a scene in a dorm at Columbia University. Taking pity on her as she is about to be kicked out by security, Jake claims her as his guest. He learns that Lainey was there in order to lose her virginity to her T.A., Matt, who Jake thinks is the most boring guy in the world. Lainey eventually realizes that Jake is also a virgin, and the two lose their virginity to each other. Years later, Lainey breaks up with her long-term boyfriend Sam after telling him she has been cheating on him. On the advice of her therapist she attends a sex addicts meeting, where she runs into Jake, who is there because of his inability to commit. Lainey schedules an appointment with Matt to end their affair; he agrees and tells her that he is engaged, but they end up having sex. On the advice of a friend she contacts Jake, and the two go on one date where they confess their sexual problems to one another. At the end of the date Jake confesses that he wants to sleep with her; however, Lainey insists that the two of them should just be friends and Jake agrees with the safe word "mousetrap" to be used to deflate sexual tension between them. The two hang out more and eventually become best friends. Strangers mistake them for a couple and their friends begin to think they are in a relationship as well. Jake learns that Lainey is going to re-enroll in medical school and possibly move to Michigan. He brings her to the birthday party of his friend's child where she meets Chris and begins seeing him to Jake's chagrin. Jake decides to move on and asks his boss Paula on a date. A date with Chris leads Lainey to run into Matt and his now-pregnant wife. At the same time Jake has sex with Paula and calls her Lainey. After their nights out Jake and Lainey spend the night together where they realize that they are both in love with one another; however, they do nothing about it. Lainey moves to Michigan and Jake devotes himself to Paula. Two months after Lainey has left, during brunch with Paula, Jake sees Matt and punches him in the face. Paula breaks up with Jake after he refers to Lainey as "my girl" and Jake is arrested. He calls Lainey from the police station and confesses he is in love with her. She tells him she feels the same way. Some time later, Matt is suing Jake for emotional distress and refuses every settlement he offers. Lainey goes to see him and tells him she will tell his wife about them if he does not drop the case. After leaving his office Lainey meets with Jake and the two rush off to be married.
After the debut of her last film, Bachelorette, Headland became what she would later define as "suicidally depressed" noting that at one point "I was literally wondering, Should I check myself into a psych ward?" To help cope with this she went to Big Sur where she "wrote all day and all night". She realized she wanted to write a movie about sex addiction after entering into a relationship with someone who was already seeing another person, recalling that at one point she drove to his house "I just started crying hysterically. I pulled over and just kept crying. It was sort of like, What is my problem right now? Is it that I'm in love with him? Is it that I feel guilty? Is it that I'm doing something I don't want to do?" From the start she did not set out to make a romantic comedy although when she realized she was making one she wanted to make sure "that it all felt very believable". For research purposes she attended sexual addiction meetings as well as read many books about the subject, including Your Brain on Sex and the relationship addiction book Leaving the Enchanted Forrest.
Sleeping with Other People received generally positive reviews from critics. On Rotten Tomatoes, the film has an approval rating of 64% based on 129 reviews, with an average score of 5.99/10. The site's critical consensus reads: "Sleeping with Other People has likable leads and flashes of inspiration, but seems unwilling or unable to surround them with the truly subversive rom-com they deserve." On Metacritic, the film has a score of 64 out of 100, based on 27 critics, indicating "generally favorable reviews". Indiewire gave the film a positive review, describing it as "a refreshingly funny and romantic feature that is more than worth falling in love with ".
